Deposition of boron from fused salt baths



United States Patent DEPOSITION OF BORON FROM FUSED SALT BATHS Hugh S.Cooper, Shaker Heights, Ohio, assignor to Walter M. Weil, ShakerHeights, Ohio No Drawing. Filed Mar. 16, 1959, Ser. No. 799,460 12Claims. (Cl. 20460) This invention relates to new and improved processesfor forming adherent elemental boron coatings on metal surfaces by fusedsalt bath electrolysis of boron oxide. Throughout this specification andthe appended claims, the term boron oxide" is intended to mean thesesqui- Oxide, B203.

The production of elemental boron by the electrolysis of fused saltbaths has been employed successfully on a commercial scale ,for a numberof years. One such process, in which the fused salt bath containedpotassium chloride or fluoride, boron oxide (B and potassium fiuoborate,is the subject of my Patent No. 2,572,249. Another such process, inwhich the fused salt bath contained only potassium fiuoborate andpotassium chloride, is the subject of my Patent No. 2,572,248.

At the time of filing my applications for those patents, and even up tothe present time, it has generally been believed, as pointed out inPatent No. 2,572,249, that, without potassium chloride or fluoride inthe bath, potassium fiuoborate would undergo slow thermal decomposition,and the bath would become viscous and gummy.

For example, I had previously found that baths, con taining aboutone-third boron oxide with the remaining two-thirds being potassiumfluoborate, were very viscons; and when electrolysis was attempted, theelectrical resistance was extremely high and the current-carryingcapacity was quite low. Also, the high operating temperatures, thenbelieved to be required, caused a considerable loss of boron oxide byvaporization and fuming. In view of all of these difliculties, Iconcluded that successful electrolysis of fused baths containingpotassium fiuoborate and boron oxide could not be achieved unlesspotassium chloride or fluoride was also present in the baths.

Surprisingly, it has now been discovered in accord ance with the presentinvention that, under certain conditions, tightly adherent boroncoatings may be produced by the electrolysis of a fused salt bathconsisting essentially of boron oxide and potassium fluoborate, withoutany potassium chloride or potassium fluoride being present in the bath.In addition, it was surprising to find that the employment of this bathprovides a number 1 of advantages not afliorded by the processes of myearlier patents. For example, boron may be deposited on the lcathode asa permanent, adherent plate. Moreover, if the process is employed as ameans for producing elemental boron, longer electrolysis periods betweencathode changes are possible since the cathode coating is more dense andadheres tightly permitting thicker deposits to be retained on thecathode. In addition, the bath may be operated at temperatures of about100 to 200 C. below those required for satisfactory use of the baths ofmy two prior patents. This is directly contrary to my prior conclusionthat higher bath temperatures would be required with the components ofthe present bath system.

One of the conditions required to achieve successful electrolysis of thebath of the present invention is that the boron oxide in the bath bemaintained below about by weight. When the boron oxide is maintainedbelow this percentage, the viscosity of the bath is substantiallyreduced and the conductivity is greatly improved. Also, the loss ofboron oxide due to fuming drops very sharply and the required minimumoperating temperature of the bath during electrolysis is appreciablylowered. The conductivity appears to improve further when the boronoxide content is reduced to about 15%, reaching a maximum conductivityat about 10%. As a result, the initial boron oxide content in the bathis preferably maintained between about 2 or 3% and 15% by weight, andany additions of make-up boron oxide during the process are preferablylimited to amounts which will not increase the boron oxide concentrationabove about 15% by weight.

The bath temperatures during electrolysis, when employing the presentinvention, may suitably be maintained in the range of about 500 and 1000C., al though highest efliciency is achieved when the baths are operatedat temperatures between about 550 and 750 C., and particularly betweenabout 600 and 700 C. The latter temperature ranges are more than C.lower than the bath temperatures generally preferred for most efiicientoperation with my prior bath systems. This means that less heat isrequired to bring the bath up to operating temperature initially and tomaintain this temperature during electrolysis. The lower operatingtemperature also results in longer crucible life, particularly wheregraphite crucibles are employed, since the dissolution of graphite bythe fused bath is accelerated at the higher temperatures.

During electrolysis, the boron oxide is electrolyzed to depositelemental boron on the cathode and release oxy gen at the anode. Thepotassium fiuoborate is believed to act only as an electricallyconductive inert vehicle, rather than as a source of boron as in theprocess of my Patent No. 2,572,248. in that prior process, the bathconsists essentially of potassium fiuoborate and potassium chloride,with the fiuoborate being electrolyzed to deposit elemental boron and toform potassium fluoride in the bath while releasing chlorine from theanode.

Since the electrolysis products formed in the process of the presentinvention, namely, elemental boron and oxygen, do not remain as a partof the bath, the only change in the bath is a depletion of the boronoxide. Thus, substantially continuous operation may be achieved by theintroduction of additional quantities of boron oxide into the bath toreplenish that being removed by electrolysis. Since periodic cathoderemoval is necessary in this case for recovery of the deposited boron,small quantities of potassium fiuoborate should also be addedoccasionally to replace fluoborate which is lost due to entrainment withthe boron deposit on the cathode, or by evaporation from the surface ofthe bath.

When the hot cathode having the boron deposit thereon is removed fromthe fused bath, a thin coating of the fused bath salts clings to thesurface. The salts quickly solidify to form a coating which protects thesurface of the boron from oxidation during cooling. If desired, thecathode may be rapidly cooled in a refrigerated atmosphere of an inertgas such as argon. In small operations, it may be more convenient toprovide additional protection against oxidation, for example, by coatingthe cathode with an inert material such as cold sodium chloride salt.After cooling, the withdrawn cathode is washed with water to remove theouter coating of solidified salts while leaving the deposited boron andthe bulk of the solidified salts entrained within the cathode depositadhering tightly to the cathode as a hard, porous composite coating. Ifremoval of the boron deposit from the cathode should be desired, thecathode with its adherent deposit may be soaked in boiling water toloosen the deposit from the cathode while dissolving most of theremainder of the entrained bath salts. Some of the cathode deposit mayfall from the cathode while this soaking in boiling water continues, andthe remainder may be readily knocked loose. The purity of the elementalboron recovered from the removed cathode deposit may be improved byleaching the deposit with strong hydrochloric acid, followed by a Waterwash to remove all traces of the acid.

The process of the present invention is particularly useful where it isdesired to plate a thin, permanently adherent coating onto a metallicobject. Boron coatings may be deposited on objects made of metals suchas iron, copper, molybdenum, nickel and other metals and alloys whichare inert to the fused bath salts by using the metal objects ascathodes. The coatings or plates formed on the objects may be of anydesired thickness, although for most purposes, thickness in the range ofabout 0.01 and 0.025 inch generally provide improved resistance tooxidation at elevated temperatures and provide improved resistance toabrasion. Although the boron coatings generally contain, dispersedthrough the coatings, a substantial proportion of entrained bath salts,these salts may be largely removed by vacuum distillation or cold Waterleaching, if their presence is detrimental for a particular application,without loosening the coating on the cathode.

One important use for the boron coated products of the present inventionis in applications where protection against atomic radiation is desired.Since boron normally contains approximately 20% of the B isotope, whichis highly opaque to neutrons emitted by radioactive materials, sheetscoated according to the method of the invention may be employed forradiation shielding purposes. The neutron shielding capacity of thecoatings may be greatly improved if the boron coatings are produced byemploying, in the fused bath, boron oxide in which the boron has beenenriched to provide a greater percentage of the B isotope.

Various aspects of the invention will be more fully understood from thefollowing examples and the accompanying discussion.

Example I About 16 kilograms of potassium fluoborate and 800 grams ofboron oxide were melted together in a graphite crucible of the typedescribed in my Patent No. 2,572,- 249 to produce a fused bathcontaining about 5% boron oxide. The graphite crucible was the anode andthe cathode was an Armco iron plate (an iron alloy containingapproximately 0.12% carbon, 0.017% manganese, 0.005% phosphorus, 0.025%silicon, and the balance, iron). After the melt reached a temperature ofabout 700 C., the current was turned on to place the cell in operation.Thevoltage ranged from about 7.4 to 7.7 volts while the current wassubstantially constant at about 700 amperes per square foot of cathodesurface throughout the run.

After about one hour of electrolysis, an additional 454 grams ofpotassium fluoborate were added, reducing the boron oxide content in thebath down to about 3%, and the electrolysis continued for one hour moreafter which the cathode was removed. The hot cathode with the borondeposit thereon was quickly coated with sodium chloride salt to preventoxidation of the boron by contact with the air during cooling. Theelemental boron deposited on the cathode was very hard and adheredtightly to the cathode. To recover the boron in the cathode deposit, thecathode was soaked in boiling wa ter for several hours to loosen thecoating. 140 grams of partially purified elemental boron were recoveredfrom one stripping of the cathode.

Thereafter, a new cathode was inserted in the bath, and 900 grams ofpotassium fluoborate and 800 grams of boron oxide were added to themolten bath, raising the boron oxide content to about 6%. Electrolysiswas started again with the voltage ranging between about 7.2 and 7.6volts and the current remaining steady at about 700 amperes per squarefoot. After two hours, an

additional 300 grams of boron oxide were added to the molten bath tocompensate for the amount decomposed during electrolysis, andelectrolysis was continued for 1%. hours more, whereupon the current wasturned oil and the second cathode removed. The deposit on the secondcathode was hard and tightly adherent and similar to that obtainedduring the first run. 265 grams of partially purified elemental boronwere recovered from a single stripping of the cathode by the procedureemployed to recover the first cathode deposit.

The following day, the bath remaining from the previous electrolysisruns was remelted and about 2700 grams of potassium fluoborate and 1000grams of boron oxide were added to bring the boron oxide content up toabout 7%. When the bath temperature reached 700 (1., a new cathode wasinserted and the current was turned on. The temperature was maintainedduring electrolysis between about 660 and 700 C. The voltage ranged fromabout 7.3 to 8.6 volts with an average current of about 600 amperes persquare foot. After four hours, 600 grams of boron oxide were added tothe molten bath to compensate for the amount decomposed duringelectrolysis, and at the end of two more hours, the run was terminated.The cathode deposit was very hard and adhered tenaciously to the cathodeas did the cathode deposits above. The elemental boron recovered asbefore from a single stripping of the cathode weighed 480 grams.

The production of the three runs above was combined and the resulting885 grams of elemental boron washed with water, with hydrochloric acid,and with water again, to remove the entrained bath salts. The resultingproduct analyzed 98% boron, 0.64% iron, and 0.20% carbon. After beingarc melted, the product analyzed 98.52% boron, 0.66% iron, and 0.35%carbon.

Example 11 About 18 kilograms of potassium fluoborate and 2 kilograms ofboron oxide were melted together in a graphite crucible of the typedescribed in Example I to form a molten bath containing approximately10% boron oxide. After the melt reached a temperature of about 700 C.,electrolysis was started using an Armco iron plate as the cathode andthe graphite crucible as the anode. The voltage was about 7 volts, andthe current was about 400 amperes per square foot. After 15 minutes, thecurrent was shut off and the cathode removed, cooled in an atmosphere ofargon, and then rinsed with water. A thin, hard, porous plate of boron(0.065 inch thick) had been deposited on the cathode. The tenacity ofthe plate was tested by striking the cathode with a steel hammer.Although the hammer impressions were visible on the surface, the platedid not crack off the cathode but remained tightly adherent thereto.

Example III A new cathode was placed in the electrolytic bath of ExampleII, and electrolysis was begun under the same conditions of bathtemperature, voltage, and current. At the end of one hour, this cathodewas removed and cooled in argon. After being rinsed with water, theplate on the cathode was measured and found to be 0.197 inch thick. Thetenacity of the plate was tested as in Example II by striking with ahammer, and the plate was found to be as tightly adherent to the cathodeas the plate in Example II.

Example IV The procedure of this example was the same as Example IIexcept that the proportion of boron oxide in the bath was increased to20%. After a 15 minute electrolysis period at about 7 volts and anaverage current of about 600 amperes per square foot, a thin, hard,porous, tightly adherent plate 0.061 inch thick similar to that producedin Examples II and III, was deposited on the cathode.v

Example V A new cathode was placed in the electrolytic bath of ExampleIV and electrolysis carried out under the same conditions. After anelectrolysis period of one hour, a hard, porous, tightly adherent plate0.155 inch thick, similar to that produced in Examples II, III, and IV,was deposited on the cathode.

From the foregoing, it will be appreciated that the present inventionprovides a new and improved process for producing tightly adherent boroncoatings on a cathode by electrolysis of a fused bath consistingessentially of boron oxide and potassium fluoborate. Not only does theprocess of the invention successfully solve the problems that previouslyprevented successful electrolysis of fused baths of boron oxide andpotassium fluoborate, but also, the process provides a number ofadvantages not heretofore attainable. The boron may be deposited on thecathode as a permanent, tightly adherent plate. Moreover, when thedeposit is to be removed from the cathode for recovery of elementalboron, longer electrolysis periods between cathode changes are possiblesince the cathode coating is more dense and adheres more tightly,permitting thicker deposits to build up on the cathode. In addition, theprocess of the invention may be operated eificiently at bathtemperatures more than 100 below those commonly used heretofore.

It is apparent from the above description of the invention that variousmodifications of the process as described can be made within the scopeof the invention. Therefore, the invention is not intended to be limitedto the details of the specific processes disclosed herein except as maybe required by the appended claims.

What is claimed is:

l. A process for producing boron from boron oxide, which compriseselectrolyzing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 20% by weight of the boron oxide.

2. A process for producing boron from boron oxide, Which compriseselectrolyzing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and by weight of the boron oxide.

3. A process for producing a permanent boron coating on a metallicobject, which comprises electrolyzing a fused salt bath consistingessentially of potassium fluoborate and between about 2% and 15% byweight of boron oxide in an electrolytic cell in which said object isthe cathode, and removing the cathode from the cell after a coating ofboron has been deposited thereon.

4. A process for producing boron from boron oxide, which compriseselectrolyzing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 15% by weight of the boron oxide inan electrolytic cell having a removable metal cathode, removing thecathode from the cell, and recovering boron deposited thereon.

5. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and by weight of the boron oxide,electrolyzing said bath in an electrolytic cell while maintaining thetemperature of the bath in the range of about 500 to about 1000 C.

6. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 15% by weight of the boron oxide,electrolyzing said bath in an electrolytic cell while maintaining thetemperature of the bath in the range of about 550 to about 750 C.

7. A process for producing a permanent boron coating on a metallicobject, which comprises preparing a fused salt bath consistingessentially of potassium fluoborate and between about 2% and 15% byweight of boron oxide, electrolyzing said bath in an electrolytic cellin which said object is the cathode, maintaining the temperature 70 6 ofthe bath in the range of about 500 to about 1000 C., duringelectrolysis, forming a tightly adherent boron deposit on the cathode,and removing the cathode from the cell after a coating of boron has beendeposited thereon.

8. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 15% by weight of the boron oxide,electrolyzing said bath in an electrolytic cell having a removable metalcathode while maintaining the temperature of the bath in the range ofabout 500 to about 1000 C., forming a tightly adherent boron deposit onthe cathode, and removing the cathode from the cell and recovering borondeposited thereon.

9. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 20% by weight of the boron oxide,electrolyzing said bath in an electrolytic cell having a removable metalcathode while maintaining the temperature of the bath in the range ofabout 500 to about 1000 C., and adding boron oxide to the bath toreplace that consumed during the electrolysis.

10. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 15% by weight of the boron oxide,substantially continuously electrolyzing said bath in an electrolyticcell having a removable metal cathode while maintaining the temperatureof the bath in the range of about 500 to about 1000 C., forming atightly adherent boron deposit on the cathode, replenishing the boronoxide as needed to maintain the boron oxide concentration betweeen about2% and 15% by weight, periodically removing the cathode from the cell,and replacing the removed cathode with a new cathode to permitcontinuation of the electrolysis.

11. A process for producing permanent boron coatings on metallicobjects, which comprises preparing a fused salt bath consistingessentially of potassium fluoborate and between about 2% and 15 byweight of boron oxide, substantially continuously electrolyzing saidbath in an electrolytic cell having at least one of said objects as thecathode while maintaining the temperature of the bath in the range ofabout 550 to about 750 C., forming a tightly adherent boron deposit onthe cathode, replenishing the boron oxide as needed to maintain theboron oxide concentration in the bath between about 2% and 15% byweight, periodically removing one of said objects from the cell andreplacing the removed object with an uncoated one to permit continuationof the electrolysis.

12. A process for producing boron from boron oxide, which comprisespreparing a fused salt bath consisting essentially of potassiumfluoborate and between about 2% and 15 by weight of the boron oxide,substantially continuously electrolyzing said bath in an electrolyticcell having a removable metal cathode while maintaining the temperatureof the bath in the range of about 550 to about 750 C., forming a tightlyadherent boron deposit on the cathode, replenishing the boron oxide asneeded to maintain the boron oxide concentration between about 2% and15% by weight, periodically removing the cathode from the cell forrecovery of the boron deposited thereon, replacing the removed cathodewith a new cathode to permit continuation of the electrolysis, andreplenishing the potassium fluoborate as needed to maintain an elfectivequantity of the fused bath in the cell.

References Cited in the file of this patent UNITED STATES PATENTS2,572,249 Cooper Oct. 23, 1951 2,810,683 Ellis Oct. 22, 1957 2,848,396Murphy et al Aug. 19, 1958

1. A PROCESS FOR PRODUCING BORON FROM BORON OXIDE, WHICH COMPRISESELECTROLYZING A FUSED SALT BATH CONSISTING ESSENTIALLY OF POTASSIUMFLUOBORATE AND BETWEEN ABOUT 2% AND 20% BY WEIGHT OF THE BORON OXIDE.